Dielectric interposer for chip to substrate soldering

ABSTRACT

A method of and device for preventing short circuits between solder joints in flip chip packaging. The dielectric interposer of the present invention has a plurality of apertures or vias which correspond to the I/O pads on a chip and substrate. Preferably, the interposer comprises a polyester film, glass, alumina, polyimide, a heat curable polymer or an inorganic powder filler in an organic material. More preferably, the interposer contains an adhesive or has adhesive layers disposed on the linear surfaces of the interposer. Cone shaped solder elements are formed within the apertures of the interposer. The dielectric interposer is positioned between a chip and substrate in an electronic module and thermally reflowed to create an electrical and mechanical interconnection. The interposer prohibits contact between the solder joints by isolating each of the joints and corresponding bonding pads. The interposer also prevents over compression of the solder joints by acting as a stand off.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Aspects of the present invention are related to subject matterdisclosed in co-pending applications entitled “Process for Forming ConeShaped Solder for Chip Interconnection,” Attorney Docket No. FI9-97-060,and “Underfill Preform Interposer for Joining Chip to Substrate,”Attorney Docket No. FI9-97-215 filed on even date herewith and assignedto the assignee of the present invention.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the manufacture of semiconductordevices, and in particular, to a method of and device for preventingshort circuits between solder joints in flip chip packaging.

[0004] 2. Description of Related Art

[0005] Multi-layer ceramic electronic components are typically joinedtogether by soldering pads on a surface of one of the electroniccomponents to corresponding pads on the surface of the other component.Controlled Collapse Chip Connection is an interconnect technologydeveloped by IBM as an alternative to wire bonding. This technology isgenerally known as C4 technology or flip chip packaging. Broadly stated,one or more integrated circuit chips are mounted above a single ormulti-layer ceramic substrate and pads on the chip are electrically andmechanically connected to corresponding pads on the substrate by aplurality of electrical connections such as solder bumps. The integratedcircuit chips may be assembled in an array such as a 10×10 array on themulti-layer ceramic surface.

[0006] Often in flip chip packaging, a semiconductor chip is joined to asubstrate by means of solder joints where an array of connections fromthe chip are connected to a corresponding array of connections on thesubstrate using solder such as a lead/tin alloy. The solder is depositedon bonding pads located on the surface of the chip and substrate bymeans of evaporation or plating and then reflowed to cause the solder towet the I/O pad. The chip is placed on the substrate and fluxed toensure good wetting between the I/O pads and the solder. The chip andsubstrate are then subjected to a higher temperature causing the solderto melt and wet each I/O pad on the package. The assembly is cooledcausing the solder to solidify, thereby providing one or more electricalconnections between the chip and the substrate forming an electronicpackage.

[0007] Subsequently, the assembly can be cleaned to receive anonconductive underfill to reduce the fatigue of the solder jointsbetween the chip and the substrate. In underfiliing the electronicpackage to provide additional stability of the solder joints, a polymerwith ceramic or glass filler is allowed to flow under the chip, betweenthe solder connections, and later heated at an elevated temperaturecausing the polymer or filled polymer to cure. In this structure andmethod, added cost in production is incurred due to the sequentialoperations of solder depositions, reflow, join, reflow, underfill, andpolymer cure.

[0008] The prior art methods of flip chip assembly do not prevent thesolder joints from being over compressed when the chip and substrate arejoined. Deformity of the solder joint can lead to short circuits whenone solder joint is in contact with an adjacent solder joint.Underfilling will not prevent the shorting since the solder-to-soldercontact has already been made. Thus, it is desirable to have a means ofpreventing short circuits between adjacent solder joints which wouldresult in increased yield and reliability of the resulting electronicmodule.

[0009] Bearing in mind the problems and deficiencies of the prior art,it is therefore an object of the present invention to provide a methodof preventing short circuits between solder joints in an electronicmodule.

[0010] It is another object of the present invention to provide anapparatus and method of fabricating the apparatus for preventing shortcircuits between solder joints in an electronic module.

[0011] A further object of the invention is to provide a method ofassembling an electronic module having enhanced yield and reliability.

[0012] It is yet another object of the present invention to provide anelectronic module having enhanced reliability.

[0013] Still other objects and advantages of the invention will in partbe obvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

[0014] The above and other objects and advantages, which will beapparent to one of skill in the art, are achieved in the presentinvention which is directed to, in a first aspect, a method offabricating an interposer for electronic packaging comprising the stepsof (a) providing a dielectric sheet; (b) forming a plurality ofapertures traversing a thickness of the sheet; and (c) forming coneshaped solder elements within the apertures.

[0015] In step (a), the dielectric sheet is preferably a polyester filmsuch as Kapton™ or Mylar™, polyimide, an inorganic powder filler in anorganic polymer, a dielectric sheet containing an adhesive, a heatcurable polymer or an inorganic material such as glass (SiO₂) or alumina(Al₂O₃).

[0016] In step (b) the plurality of apertures are preferably formed byphotolithography, mechanical punching, precision drilling, or laserablation.

[0017] In step (c) the cone shaped solder elements are formed byinjection molding, or by placing solder balls into the apertures andcoining the solder balls with a die having cone shaped cavities.

[0018] The method may further include the step of coating the solderelements with tin or coating the solder elements with a second solderhaving a lower melting temperature than the solder elements.Additionally, the step of attaching an adhesive layer to a linearsurface of the dielectric sheet may be included as well.

[0019] The present invention is directed to, in a second aspect, aninterposer for chip to substrate joining comprising a dielectric sheethaving a plurality of vias, the vias traversing a thickness of thesheet; and conical solder elements deposited within the vias of thesheet.

[0020] The dielectric sheet preferably comprises an organic polymer,polyimide, a polyester film such as Kapton™ or Mylar™, an inorganicpowder filler in an organic polymer, a dielectric sheet containing anadhesive, a heat curable polymer or an inorganic material such as glass(SiO₂) or alumina (Al₂O₃).

[0021] The vias in the dielectric sheet are preferably formed byphotolithography, mechanical punching, precision drilling, or laserablation.

[0022] The cone shaped solder elements are preferably injection moldedor coined solder balls deposited in the vias coined with a die havingcone shaped cavities. The solder elements may be coated with tin orcoated with a second solder having a lower melting temperature than thesolder elements.

[0023] The interposer may further include an adhesive sheet havingcorresponding apertures to the vias disposed on a linear surface of thedielectric sheet.

[0024] The present invention is directed to, in still yet anotheraspect, a method of assembling electronic modules comprising the stepsof: (a) providing a semiconductor chip; (b) providing a substrate formounting the chip; (c) providing an interposer comprising a dielectricsheet having a plurality of apertures, the apertures traversing athickness of the sheet; and conical solder elements deposited within theapertures of the sheet; (d) aligning the interposer between the chip andthe substrate; and (e) thermally reflowing the solder elements creatingan electrical connection between the chip and the substrate.

[0025] In step (c) the dielectric sheet comprises an organic polymer,polyimide, a polyester film such as Kapton™ or Mylar™, an inorganicpowder filler in an organic polymer, a heat curable polymer, or aninorganic material such as glass (SiO₂) or alumina (Al₂O₃).

[0026] Wherein in step (c) the dielectric sheet comprises a heat curablepolymer, step (e) produces an underfilled electronic module. In step (d)the solder elements are aligned to corresponding bonding pads onsurfaces of the chip and the substrate.

[0027] The method may further include forming an adhesive layer on asurface of the interposer, and prior to step (d), the step of coatingthe conical solder elements with a lower melting solder than the solderelements or with tin.

[0028] The present invention is directed to, in still yet anotheraspect, a flip chip electronic module comprising a semiconductor chip; asubstrate for mounting the chip; and a dielectric interposer disposedbetween the chip and substrate having a plurality of apertures, theapertures traversing a thickness of the interposer, and conical solderelements deposited within the apertures, wherein the module is thermallyreflowed such that the chip and the substrate are electrically andmechanically interconnected by the solder elements and the solderelements are not in contact with an adjacent solder element.

[0029] The solder elements of the module may be coated with tin or alower melting temperature solder than the solder elements.

[0030] The dielectric interposer may comprise of an organic polymer,polyimide, a polyester film such as Kapton™ or Mylar™, an inorganicpowder filler in an organic polymer, a polymer with an adhesive, a heatcurable polymer, or an inorganic material such as glass (SiO₂) oralumina (Al₂O₃). The interposer may further include adhesive layersdisposed on the linear surfaces of the interposer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The features of the invention believed to be novel and theelements characteristic of the invention are set forth withparticularity in the appended claims. The figures are for illustrationpurposes only and are not drawn to scale. The invention itself, however,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

[0032]FIG. 1 is a top plan view of a substrate of the prior art.

[0033]FIG. 2 is an elevational cross-sectional view of an electronicmodule of the prior art.

[0034]FIG. 3 is a top plan view of an interposer of the presentinvention.

[0035]FIG. 4 is a top plan view of an interposer of the presentinvention aligned with a substrate.

[0036]FIG. 5 is an elevational cross-sectional view of an electronicmodule utilizing the interposer of the present invention.

[0037]FIG. 6 is an elevational cross-sectional view of a preferredembodiment of the interposer of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0038] In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-6 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

[0039] In FIG. 1 there is shown a top plan view of a substrate 10 of theprior art used in flip chip packaging. On the surface of the substrate10 are bonding pads 14 which correspond to the I/O pads on the substrateto be electrically and mechanically connected to corresponding I/O padson a chip. Solder balls 17 are formed on substrate 10 as theinterconnections. Normally, the solder balls 17 flow within theboundaries specified by the bonding pads 14, however, solder balls andbonding pads are often aligned such that shorts 19 occur as shown inFIGS. 1 and 2. FIG. 2 is a partial cross-sectional view of an electronicmodule having substrate 10 with bonding pads 14 a, connected to chip 12having corresponding bonding pads 14 b by solder bumps 17. If chip 12 isplaced on substrate 10 during formation of the flip chip package withexcessive force, there can be an over compression of the solder bumps17. This over compression can lead to shorts in area 19 wherein thesolder bumps 17 are misshapen to such a degree that two solder bumps cancome in contact with each other.

[0040]FIG. 3 shows a free-standing polymer interposer 30 of the presentinvention. Preferably, the interposer 30 is made from a dielectricmaterial 33. Examples include polyester films such as Kapton™ or Mylar™,organic polymers such as polyimide, an inorganic powder filler in anorganic polymer, ceramic, inorganic materials such as glass (SiO₂) oralumina (Al₂O₃), or any other appropriate material which isnon-conductive and which may or may not be heat curable. The interposer30 has a plurality of apertures or vias 35 which correspond to the I/Opads on a chip 42 and substrate 40 in the resultant electronic packageshown in FIGS. 4 and 5. For the ease of differentiation in the drawings,the apertures 35 are shown as circles and bonding pads 44 are shown assquares although any shape may be utilized on practicing the invention.

[0041]FIG. 4 is a top plan view of the electronic package utilizing theinterposer 30 of the present invention. Interposer 30 can be made byobtaining a dielectric sheet correctly sized for the resultantelectronic module. Apertures or vias are formed in the dielectric sheetby such means as mechanical punching, precision drilling, laserablation, or any other means which allow apertures of very smalldimensions to be formed.

[0042] Solder can be screen printed into the apertures of the interposeror solder balls can be injection molded or placed into the apertures.Solder balls placed into the apertures of the dielectric sheet must thenbe mechanically pressed or thermally processed such that the solderballs will remain in the interposer. The solder of the solder paste maycontain flux in the paste or have the flux deposited on the surface ofthe paste for the purpose of improving the wettability between thesolder and each I/O pad to be electrically connected. The flux can be ofthe variety which leaves minimal residue so that no cleaning is requiredsubsequent to joining the chip and the substrate. However, the use offlux which requires cleaning after joining is not required whenpracticing this invention.

[0043] Once the interposer has been fabricated and solder joints formedor placed into the apertures, the interposer is placed between the chipand the substrate as shown in FIGS. 4-6. The assembly can be thermallyreflowed to create an electrical interconnection between the chip andsubstrate. Preferably, the interposer may comprise a glass or ceramicfilled dielectric sheet having adhesive layers on its surface such thatnot only is an electrical interconnection formed, but a mechanicalenhancement is also produced.

[0044] An electronic module utilizing the interposer 30 of the presentinvention having substrate 40 and chip 42 both having correspondingbonding pads 44 a and 44 b, is shown with more detail in FIG. 5. Area 50between two adjacent solder joints 37 are no longer in contact to shortout the electronic module. The interposer 30 successfully prohibitscontact between solder joints 37 by isolating each of the solder jointsand corresponding bonding pads. The interposer 30 also prevents overcompression of the solder joints 37 by acting as a stand off between thesubstrate 40 and chip 42. In another preferred embodiment, theinterposer may comprise a heat curable polymer which also acts as anunderfill such that only one thermal activation step is required toreflow the solder and join the components of the electronic package.

[0045] In a preferred embodiment, the interposer 630 of FIG. 6 comprisesa dielectric sheet 633 having a plurality of apertures. The dielectricsheet 633 further comprises adhesive layers 635 on its surface whichfacilitate joining between the interposer and the chip, and theinterposer and the substrate. Of course, the dielectric sheet of theinterposer may also be a polymer with an adhesion promoter containedtherein such that the polymer sheet itself has enhanced adhesivecharacteristics and could also flow around the solder joints. Such adielectric interposer could be heated during or after formation of thesolder joints to electrically connect the chip to the substrate.Alternatively, such a dielectric interposer with adhesive can enhancethe mechanical integrity and reliability of the interconnection of theelectronic module. The adhesive may also be deposited after interposerfabrication but prior to creating the chip to substrate interconnection.

[0046] The preferred embodiment of an interposer of the presentinvention contains cone shaped solder 637 disposed within the aperturesof the polymer sheet 633. The cone shape solder 637 permits a reducedforce for a given I/O pad to allow for some non-planarity between thechip, the interposer, and the substrate. The solder could be shaped witha coining die made from graphite, glass, molybdenum, titanium or analloy thereof, nickel alloy, or stainless steel. Other methods ofshaping the solder include thermal injection and acooling/solidification process.

[0047] The solder composition used in the interposer of the presentinvention may also comprise of more than one solder composition. Forexample, a high melt solder composition such as a 95-97% lead/tin alloycould be used with a lower melting solder such as a eutectic lead/tinalloy. In this case, the combination of high melt and low melt solderwould provide a means to create an interconnection at a moderate joiningtemperature. Alternatively, the solder joints may also be coated withtin or the lower melting solder during formation of the interposer priorto placing the interposer between a chip and substrate.

[0048] The present invention achieves the objects recited above. Thepresent invention successfully reduces short circuiting between solderjoints in an electronic module by isolating the solder joints. Placingthe interposer between the chip and the substrate in an electronicmodule also acts as a stand off reducing solder to solder electricalshorts and can provide improved mechanical integrity for chip tosubstrate or chip to board interconnections. Thus, resulting electronicmodules utilizing an interposer of the present invention has enhancedreliability and integrity.

[0049] While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. A method offabricating an interposer for electronic packaging comprising the stepsof (a) providing a dielectric sheet; (b) forming a plurality ofapertures traversing a thickness of said sheet; and (c) forming coneshaped solder elements within said apertures.
 2. The method of claim 1wherein in step (a) said dielectric sheet comprises a polyester film. 3.method of claim 1 wherein in step (a) said dielectric sheet comprisesglass.
 4. method of claim 1 wherein in step (a) said dielectric sheetcomprises alumina.
 5. The method of claim 1 wherein in step (a) saiddielectric sheet comprises an organic polymer.
 6. The method of claim 1wherein in step (a) said dielectric sheet comprises a heat curablepolymer.
 7. The method of claim 1 wherein in step (a) said dielectricsheet comprises polyimide.
 8. The method of claim 1 wherein in step (a)said dielectric sheet comprises an inorganic powder filler in an organicmaterial.
 9. The method of claim 1 wherein in step (a) said dielectricsheet contains an adhesive.
 10. The method of claim 1 wherein in step(b) said plurality of apertures are formed by photolithography.
 11. Themethod of claim 1 wherein in step (b) said plurality of apertures areformed by mechanical punching.
 12. The method of claim 1 wherein in step(b) said plurality of apertures are formed by precision drilling. 13.The method of claim 1 wherein in step (b) said plurality of aperturesare formed by laser ablation.
 14. The method of claim 1 wherein in step(c) said cone shaped solder elements are formed by injection molding.15. The method of claim 1 wherein in step (c) said cone shaped solderelements are formed by placing solder balls into said apertures andcoining said solder balls with a die having cone shaped cavities. 16.The method of claim 1 further including the step of coating said solderelements with tin.
 17. The method of claim 1 further including the stepof coating said solder elements with a second solder having a lowertemperature than said solder elements.
 18. The method of claim 1 furtherincluding the step of attaching an adhesive layer to a linear surface ofsaid dielectric sheet.
 19. An interposer for chip to substrate joiningcomprising a dielectric sheet having a plurality of vias, said viastraversing a thickness of said sheet; and conical solder elementsdeposited within said vias of said sheet.
 20. The interposer of claim 19wherein said dielectric sheet comprises an organic polymer.
 21. Theinterposer of claim 19 wherein said dielectric sheet comprisespolyimide.
 22. The interposer of claim 19 wherein said dielectric sheetcomprises a polyester film.
 23. The interposer of claim 19 wherein saiddielectric sheet comprises glass.
 24. The interposer of claim 19 whereinsaid dielectric sheet comprises alumina.
 25. The interposer of claim 19wherein said dielectric sheet comprises an inorganic powder filler in anorganic polymer.
 26. The interposer of claim 19 wherein said dielectricsheet comprises a heat curable polymer.
 27. The interposer of claim 19wherein said dielectric sheet contains an adhesive.
 28. The interposerof claim 19 wherein said plurality of vias are formed byphotolithography.
 29. The interposer of claim 19 wherein said pluralityof vias are formed by mechanical punching.
 30. The interposer of claim19 wherein said plurality of vias are formed by precision drilling. 31.The interposer of claim 19 wherein said plurality of vias are formed bylaser ablation.
 32. The interposer of claim 19 wherein said cone shapedsolder elements are injection molded.
 33. The interposer of claim 19wherein said cone shaped solder elements are solder balls deposited intosaid vias and coined with a die having cone shaped cavities.
 34. Theinterposer of claim 19 wherein said solder elements are coated with tin.35. The interposer of claim 19 wherein said solder elements are coatedwith a second solder having a lower temperature than said solderelements.
 36. The interposer of claim 19 further including an adhesivesheet having corresponding apertures to said vias disposed on a linearsurface of said dielectric sheet.
 37. A method of assembling electronicmodules comprising the steps of: (a) providing a semiconductor chip; (b)providing a substrate for mounting said chip; (c) providing aninterposer comprising a dielectric sheet having a plurality ofapertures, said apertures traversing a thickness of said sheet; andconical solder elements deposited within the apertures of said sheet; p1(d)aligning the interposer between said chip and said substrate; and(e)thermally reflowing said solder elements creating an electricalconnection between said chip and said substrate.
 38. The method of claim37 wherein in step (d) said solder elements are aligned to correspondingbonding pads on surfaces of said chip and said substrate.
 39. The methodof claim 37 further including forming an adhesive layer on a surface ofsaid interposer.
 40. The method of claim 37 wherein in step (c) saiddielectric sheet comprises an organic polymer.
 41. The method of claim37 wherein in step (c) said dielectric sheet comprises polyimide. 42.The method of claim 37 wherein in step (c) said dielectric sheetcomprises a polyester film.
 43. The method of claim 37 wherein in step(c) said dielectric sheet comprises glass.
 44. The method of claim 37wherein in step (c) said dielectric sheet comprises alumina.
 45. Themethod of claim 37 wherein in step (c) said dielectric sheet comprisesan inorganic powder filler in an organic polymer.
 46. The method ofclaim 37 wherein in step (c) said dielectric sheet comprises a heatcurable polymer.
 47. The method of claim 37 further including, prior tostep (d), the step of coating said conical solder elements with a lowermelting solder than said solder elements.
 48. The method of claim 37further including, prior to step (d), the step of coating said conicalsolder elements with tin.
 49. The method of claim 37 wherein in step (c)said dielectric sheet comprises a heat curable polymer and wherein step(e) produces an underfilled electronic module.
 50. A flip chipelectronic module comprising a semiconductor chip; a substrate formounting said chip; and a dielectric interposer disposed between saidchip and substrate having a plurality of apertures, said aperturestraversing a thickness of said interposer, and conical solder elementsdeposited within said apertures, wherein said module is thermallyreflowed such that said chip and said substrate are electrically andmechanically interconnected by said solder elements and said solderelements are not in contact with an adjacent solder element.
 51. Themodule of claim 50 wherein said solder elements are coated with tin. 52.The module of claim 50 wherein said solder elements are coated with alower melting temperature solder than said solder elements.
 53. Themodule of claim 50 wherein said dielectric interposer comprises apolyester film.
 54. The module of claim 50 wherein said dielectricinterposer comprises glass.
 55. The module of claim 50 wherein saiddielectric interposer comprises alumina.
 56. The module of claim 50wherein said dielectric interposer comprises polyimide.
 57. The moduleof claim 50 wherein said dielectric interposer comprises an inorganicpowder filler in an organic polymer.
 58. The module of claim 50 whereinsaid dielectric interposer comprises a heat curable polymer.
 59. Themodule of claim 50 wherein said dielectric interposer comprises apolymer containing an adhesive.
 60. The module of claim 50 wherein saidinterposer further includes adhesive layers disposed on linear surfacesof said interposer.